1. Field of the Invention
The present invention relates to the use of lipocalin-2 as a diagnostic marker for heart risk, chronic heart disease and stroke risk. Specifically, the present invention relates to the evaluation of risk and progression of these risks by measuring the concentration of circulating lipocalin-2 in a subject and comparing the measured level to lipocalin-2 levels within a standardized or standard population.
2. Background
Cardiovascular disease remains the most common cause of morbidity and mortality in the developed world. Therefore, prevention of cardiovascular disease is an area of major public health importance. Currently, several risk factors for future cardiovascular disease have been described and are in wide clinical use in the detection of individuals at high risk, such as evaluations of total and HDL cholesterol levels. However, a large number of cardiovascular diseases occur in individuals with apparently low to moderate risk profiles, and the ability to identify such patients is limited. Moreover, accumulating data suggests that the beneficial effects of certain preventive and therapeutic treatments for patients at risk for or known to have cardiovascular disease differ in magnitude among different patient groups. At this time, however, data describing diagnostic tests to determine whether certain therapies can be expected to be more or less effective are lacking.
Lipocalin-2, also known as 24p3 (1) and neutrophil gelatinase-associated lipocalin (NGAL) (2), is a 25 kDa secretory glycoprotein that was originally identified in mouse kidney cells and human neutrophil granules. It belongs to the lipocalin superfamily that consists of over 20 small secretory proteins, including RBP4, fatty acid binding proteins (FABP), major urinary proteins (MUP), apolipoprotein D (apoD) and prostaglandin D synthases (PGDS) (3). The common feature of this protein family is their capacity to bind and transport small lipophilic substances, such as free fatty acids, retinoids, arachidonic acid, and various steroids (4). Although it has previously been reported that lipocalin-2 binds weakly with leukotriene B4 and lipopolysaccharides (5), its high affinity endogenous ligands remain to be identified. In addition to neutrophils, lipocalin-2 is expressed in several other tissues, including liver, lung, kidney, adipocytes, and macrophages (6-8). Several inflammatory stimuli, such as lipopolysaccharides and “IL-1β”, can markedly induce lipocalin-2 expression and secretion in these cells (6, 9). Notably, the pro-inflammatory transcription factor NF-kappaB has been shown to transactivate lipocalin-2 expression through binding with a consensus motif within the promoter region of the lipocalin-2 gene (10), suggesting that this secretory protein might be involved in the inflammatory responses.
Elevated plasma lipocalin-2 levels was recently found associated with increased proteolytic activity in atherosclerotic lesions and implicated as renal failure following ischemic injury, cisplatin nephrotoxicity, or infection (11-15). The partial association of lipocalin-2 with matrix metallopeptidase 9 suggests that lipocalin-2 may exert modulatory actions on MMP9 by protecting MMP9 from degradation (2, 16), and preserving its enzymatic activity. A recent study demonstrated MMP9 and lipocalin-2 co-localized in macrophages and smooth muscle cells (SMC) in human atherosclerotic plaques. In situ zymphography showed higher MMP9 activity where lipocalin-2 are more expressed, indicating the potential role of lipocalin-2 in modulating MMP9 activity and destabilizing plaque (15).